Former Russian spy and anti-Putin critic, Alexander Litvinenko has died in a London hospital, the victim of an apparent poisoning.

The story of the death of Alexander Litvinenko reads like a tale straight out of the Cold War and from the murky world of espionage, it leaves more questions than answers.

The former Russian Federal Security Service (FSB) agent fled to Britain in 2000 where he was granted asylum.

Litvinenko had been investigating the recent death of dissident Russian writer Anna Politkovskaya and on November 1st he had at least two meetings in central London, one with a former KGB bodyguard, the other with an Italian academic security expert.

Shortly after, he said he started feeling ill.

Eventually, he was transferred to London's University College Hospital. Earlier this week, his condition worsened. Hospital spokesman Jim Down then broke the news of Mr. Litvinenko's death to the awaiting reporters outside.

"We are sorry to announce that Alexander Litvinenko died at University College Hospital (UCH) at 9:21 on the 23rd of November 2006," he said. "He was seriously ill when he was admitted to UCH on Friday, November the 17th and the medical team at the hospital did everything possible to save his life. On Sunday evening, he was transferred to the intensive care unit where he could be closely monitored and received any critical support he needed."

At first, it was thought the deadly chemical thallium was the cause, but that was later dismissed. Radioactive medicine used in chemotherapy was also considered, but again that was not established.

Hospital spokesman Jim Down says the mystery continues.

"Every avenue was explored to establish the cause of his condition and the matter is now an ongoing investigation being dealt with by detectives from New Scotland Yard," he said.

It is hoped an autopsy may provide important clues as to the exact cause. Meanwhile, the investigation by Britain's anti-terrorism squad goes on. The Kremlin has repeatedly denied any involvement in what it calls, a tragic case.